A standard feature of integrated circuit manufacture is the use of step and repeat cameras during photolithography. Since a semiconductor wafer is far too large for it to be possible to project a single image, that is in focus and free of distortion, over the entire surface, only a small part of the final pattern (called a reticle) is exposed at any given time. This sub-image is repeated many times by the step and repeat camera which moves with great precision from one location to another until the full pattern on the wafer has been exposed.
These repeated exposures directed through the lens of the step and repeat camera cause it to steadily heat up as stepping and exposing proceed. Since the optical tolerances in systems of this type are extremely tight, even the relatively gentle heating of the lens in this way (estimated to be between about 0.05 and 0.3.degree. C.) is sufficient to introduce a number of problems:
1. focus drift PA1 2. magnification drift PA1 3. distortion PA1 4. curvature of field
Problems 1 and 2 can be overcome by appropriate adjustment of the system. Thus, the location of the resist surface can be moved to coincide with the new focal plane and the magnification of the system can be increased or decreased, as needed to compensate for the heating effect. The manufacturers of such systems are well aware of the lens heating problem. In FIG. 1 we show a curve of the focus correction recommended by a manufacturer as a function of time, while step and repeat image projection is taking place. The effects of lens heating are first felt after about 15 minutes (in this case), following which the repeated increase and decrease in the lens temperature can clearly be seen. After about 40 minutes the lens temperature had reached its maximum and the run was terminated.
While the curve of FIG. 1 is useful for illustrating the lens heating effect, it is of greater value to users for the manufacturer to provide data that can be used to drive the software that controls the system. In general, such data takes the form of two scaling constants .mu..sub.1 and .mu..sub.2 expressed in microns per watt, and two time constants .tau..sub.1 and .tau..sub.2.
Problem b 3 is relatively minor but problem 4 (curvature of field) can introduce serious difficulties. It can be eliminated, or at least minimized, only by proper design of the original lens system and, should it occur, cannot be compensated for in the manner already described for dealing with problems 1 and 2. Even in the latter case, while the time constants are valid over a wide range of operating conditions, they depend on the lens quality and the cooling time. The focus drift will vary somewhat as factors such as reticle pattern, exposure sequence, exposure energy, and light intensity change.
Thus, additional information, beyond what the manufacturer has provided, needs to be gathered by the user if he is to get maximum performance from the projection system. In particular, the extent to which curvature of field is a problem under the intended operating conditions needs to be determined and correction factors for the manufacturer's scaling constants need to be measured.
In the prior art this has been achieved by modelling the lens cooling curve as measured by an image sensor combined with the compensated level sensor height. While this method enables the lens heating machine constants to be fine-tuned very accurately, its implementation is both time consuming and relatively expensive. As will be described below, the present invention makes possible the measurement of these constants in a short time at a minimal cost. The present invention also makes it possible to quickly determine the extent to which a given projection system is subject to curvature of field as a result of lens heating.
We have been unable to find any prior art that teaches the approach taken by the present invention, but note the following to be of interest. Dirksen et al. (U.S. Pat. No. 5,674,650 October 1997) evaluate the performance of a step and repeat projection system by means of an image sensor in conjunction with a set of test marks. Krivokapic et al. (U.S. Pat. No. 5,655,110 August 1997) teach a method for analyzing and evaluating the contribution of various process parameters to the quality of the final image. This makes it possible to concentrate process control in the areas where there is the most leverage.